1. Field of the Invention
The present invention relates to a metal mask, and more particularly, to a mask and a mask frame assembly to vacuum evaporate a thin layer for an organic electroluminescent device.
2. Description of the Related Art
In general, electroluminescent devices are active luminescent display devices. Electroluminescent devices have been noted as the next generation display devices because they have a wide viewing angle, high contrast, and high response speed.
Electroluminescent devices are divided into inorganic electroluminescent devices and organic electroluminescent devices according to a material of a luminescent layer. Organic electroluminescent devices have a higher luminance and response speed than that of inorganic electroluminescent devices and are capable of displaying color images.
Generally, conventional organic electroluminescent devices comprise a first electrode which is formed on a transparent insulation substrate, in a predetermined pattern, an organic luminescent layer which is formed on the insulation substrate having the first electrode through a vacuum evaporation, and a second electrode, i.e., a cathode electrode, which is formed on a top surface of the organic luminescent layer to cross the first electrode.
Typically, the first electrode is made of indium tin oxide (ITO), which is patterned by being wet etched using an etchant containing ferric chloride, according to a photolithographic method. However, where the second electrode, i.e., the cathode electrode, is etched using the photolithographic method, and where a resist is lifted and the second electrode is etched, moisture permeates through the interface between the organic luminescent layer and the second electrode. The permeation of moisture remarkably decreases the life span and performance of the organic electroluminescent device.
To overcome this problem, methods of evaporating an organic electroluminescent material for an organic luminescent layer and a material for a second electrode have been proposed. In manufacturing organic electroluminescent devices using these evaporation methods, a first electrode is formed of, for example, ITO, on a transparent insulation substrate in a stripped pattern using, for example, a photolithographic method. Next, an organic luminescent layer is stacked on the transparent insulation substrate having the first electrode. Thereafter, a mask having the same pattern as a second electrode is placed on the organic luminescent layer, and a material for the second electrode is evaporated to form the second electrode.
A mask which is used to form an organic luminescent layer or a second electrode, i.e., a cathode electrode, through an evaporation method, an organic electroluminescent device manufactured using the mask, and a manufacturing method thereof are disclosed in Korean Patent Publication No. 2000-60589. The mask is structured to have slots formed in a main body of a thin plate, the slots being spaced apart by a predetermined distance in a stripped pattern.
A mask including slits and bridges forming a mesh in a thin metal plate is disclosed in Korean Patent Publication No. 1998-71583.
A mask including an electrode mask portion and a pair of terminal mask portions is disclosed in Japanese Patent Publication No. 2000-12238. The electrode mask portion includes marking portions which have a width corresponding to a gap between second electrodes and are parallel with each other in a stripped pattern, and connection portions which connect both ends of each marking portion.
As described above, in the conventional masks, slit-shaped holes are formed in a thin metal plate in a stripped pattern. Accordingly, although the thin metal plate is supported by a frame at its edge so that tension is applied to the frame, the mask sags due to slots formed in the mask. Therefore, the conventional masks do not closely contact a substrate. This problem becomes more critical as the size of the substrate increases. In addition, during evaporation, the mask expands due to heat, which increases the degree of sagging.
FIG. 1 shows an example of a conventional mask for mass production of electroluminescent devices. The mask 10 comprises a single thin metal plate 11 having a plurality of unit mask patterns 12 so as to form a plurality of organic electroluminescent devices through evaporation, at one time. The mask 10 is fixed to and supported by a frame 20 such that a tension is applied to the mask 10.
Since the mask 10 used to mass produce the electroluminescent devices is large, sagging of the mask 10 is prevalent even though the mask 10 is supported by the frame 20 with uniform tension. In addition, it is necessary to weld the mask 10 to the frame 20 so as to maintain the width of each slot 12a formed in the unit mask patterns 12 within a predetermined tolerance range. Here, where the tension is uniformly distributed in the mask 10 to prevent the mask 10 from sagging, distortion often occurs in the pitch of the slots 12a in the masking patterns 12, making it difficult to maintain the predetermined tolerance range. In particular, where a slot in a unit mask pattern 12 at a particular position in the mask 10 is deformed, all slots adjacent to the deformed slot are also deformed, so the slots move relative to a substrate and go beyond the predetermined tolerance range. This phenomenon becomes more accentuated in a tangential direction of each slot (an orthogonal direction to the lengthwise direction of each slot).
For example, where the unit mask patterns 12 are distorted, a total pitch becomes large, so red, blue, and green organic layers cannot be formed with precision, on individual unit electrode patterns on a substrate. Since the adjustment of the pitch of each unit mask pattern 12 and the total pitch is extremely restricted, there is a limit in increasing the size of the mask 10.
FIG. 2 shows a conventional mask 10 that is fixed to a frame 20 using tension that acts at each side of the mask 10. As shown by dashed lines of FIG. 2, support bars 21 at the right and left sides of the frame 20 are curved inward due to the tension of the mask 10, and support bars 22 at the upper and lower sides of the frame 20 are curved outward. In another case, FIG. 3 illustrates that support bars 21 at right and left sides of a frame 20 are curved outward, and support bars 22 at upper and lower sides of the frame 20 are curved inward.
Accordingly, even where the mask 10 is welded to the frame 20 with uniform tension, it is difficult to adjust the total pitch to correct the deformation of unit mask patterns and a difference between a unit electrode pattern and a unit mask pattern.
A mask reported to overcome a problem associated with creep in strips that define slots, due to a thermal expansion of the mask, is disclosed in Japanese Patent Publication No. 2001-247961. The mask is used to form a patterning layer on a substrate through evaporation and is composed of a mask portion, in which a plurality of first openings are separated by barriers, and a screen portion, in which a plurality of second openings having a smaller opening area than the first openings are disposed directly above each first opening.
A structure of a magnetic mask is disclosed in Japanese Patent Publication No. 2001-273979. An evaporation mask-frame which includes a mask pattern corresponding to an evaporation area and which is closely adhered to a substrate on which evaporation is to be performed, so as to mask the evaporation area, is disclosed in Japanese Patent Publication No. 2001-254169. The mask pattern includes a fine pattern unit having fine gaps, which are difficult to manufacture accurately. The fine pattern unit is supported by a fine rib.
These conventional masks are made of a magnetic material so as to allow the masks to be closely adhered to a substrate. However, a pitch between strips changes due to the weight and the tension of a mask, and a total pitch also changes due to the internal stress of a mask and a frame.
A mask to prevent thermal deformation of an opening and to increase the accuracy thereof is disclosed in Japanese Patent Publication No. 2002-235165 and U.S. Pat. No. 3,241,519. A frame and a mask, on which a plurality of unit patterns are formed for a pattern of a large display, are disclosed in EP Publication No. 1,209,522 A2. Similar masks to that are described above are also disclosed in U.S. patent application Ser. No. 2002/0025406 A1. However, these masks do not solve some or all of the above-described problems.
A mask frame assembly having a plurality of masks which are supported by a single frame, in which an opening corresponding to each mask is formed, is disclosed in EP Publication No. 1,229144 A2. However, there is a limit in narrowing a distance between masks. Accordingly, a substrate, on which evaporation is performed, is wastefully used. Moreover, assembling the masks together is complicated, and the mask frame assembly cannot be used to form a pattern of a large display.